Thomas's original ambition was to be a professional footballer. He achieved a place on the ground staff at Norwich City, playing as an inside forward. However an ankle injury forced him to give up playing. He was then persuaded to take up refereeing at the age of sixteen. He made rapid progress, reaching the Welsh League and in 1964 became a Football League linesman. Two years later aged only thirty he became a Football League referee, one of the youngest referees of the time .
Thomas officiated in both the 1974 and 1978 World Cups, and in the 1976 European Championship. During a long and sometimes controversial career as a referee in the old English First Division he was known as "The Book" for his strict interpretation of the laws of the game.
At the European Championship of 1976 in Yugoslavia, Thomas was the referee of the semi-final between Czechoslovakia and the Netherlands. In the extra time of that match, he missed a rough foul by Antonín Panenka on Johan Cruyff, after which Zdenek Nehoda was able to score the decisive goal. Willem van Hanegem refused to play on after the incident and received a red card. In a 2008 documentary by the Dutch TV program Andere Tijden Sport, Thomas admitted he made a mistake by not noticing the foul.
Two years later, at the World Cup in Argentina, he officiated the Round 1 match between Brazil and Sweden, in which he infamously blew the whistle for full time during a play from a corner kick, thus disallowing the late goal Zico scored moments later, which would have given Brazil a 2-1 win.
In 1984, he published his autobiography, By the Book.
- Clive Thomas (1984) By the Book, Collins Willow, p3
- Confirmation of place of residence during career: example from Welsh Football Data Archive website.
- Clive Thomas: "Mijn fout kostte Oranje EK-goud" Archived 25 January 2009 at the Wayback Machine (in Dutch)
- Autobiography, "By the Book": TMWMTT.com website.
- High Sheriff of Mid Glamorgan, 2004: BBC.co.uk website.